Several different designs of apparatus of the kind defined in the introduction are known to the art. Apparatus on which the present invention is based will be described summarily hereinafter with reference to FIGS. 1 and 2.
In this known construction, a chamber window that delimits the measuring chamber is comprised of a clear glass plate and the total chemiluminescence generated in the measuring chamber is measured.
It is also known, by way of an alternative, to incorporate a filter in the chamber window or to provide the window with a filter. The filter used in this respect is one that will extinguish light wave lengths shorter than 620 nm. Such a filter is used in NO/NO.sub.2 analysis processes.
It is therewith known to deliver the gas mixture and the ozone gas continuously to the measuring equipment and particularly to the measuring chamber, through a respective hose-mounted throttle valve. Alternatively, the gas mixture can be delivered discontinuously, by controlling a number of valves in accordance with the expiration cycle.
The International Patent Publication WO 95/02181 (PCT/SE 94/00659) teaches a system of establishing the NO-content of the expiration air from a living being.
Since the present invention is based on the ability to evaluate relatively small amounts of nitrogen oxide (NO) present in a gas mixture, it can be mentioned that it is also known to evaluate the nitrogen oxide content of a gas with the aid of chemiluminescence, with the aid of, for instance, NO/NO.sub.2 -analyzers used by Monitor Labs, 8840, and marketed by Oleico AB, Lidingo, Sweden.
The presence of nitrogen oxide (NO) can also be shown by collection in distilled water to which iron (II) sulphate has been added, freeze-drying the mixture to dryness and thereafter shown as nitrite, either with the Diazo-reaction according to Martin, et al or with a Nitur-test qualitative nitrite stick.
Other known measuring instruments that operate with mass-spectrography can be used for this purpose.
A common feature of all earlier known methods of evaluating the NO-content of a gas mixture is that it is difficult to evaluate the time-wise short variation of the NO-content, for instance during a single expiration cycle.